Number portability NP is a service where the subscriber can keep his old telephone number within a portability cluster and thus retain easy contactability. In a fixed network, number portability means in that the subscriber can change his location and even his operator and still have the same phone number. In mobile networks, number portability means that a mobile subscriber can change his subscription network without changing his telephone number for terminating traffic. One major feature of NP is that it should have no influence on the services offered. In other words, regardless of whether the number of a subscriber is ported or not, the services offered to the subscriber should be the same. A ported number means a number that has undergone a porting process (i.e. transfer) between locations, operators or subscription networks.
In wireless networks, number portability is usually implemented with a Mobile Number Portability Signalling Relay Function (MNP-SRF) and/or as an IN-based solution e.g. with a CoreINAP interface. In fixed networks, number portability is usually implemented as an IN-based solution. In mobile number portability, the MNP-SRF is mandatory for non-call-related signalling. An intelligent network IN is a network architecture attached to a basic network (fixed or mobile network, for example), which enables faster, easier and more flexible implementation and control of services. This is achieved by moving service control away from the switching centre to a separate functional unit in the intelligent network.
A problem arises when an IN service of the calling subscriber utilizes the number range of a B-number, i.e. the called telephone number, for determining a service parameter, such as tariff level. Because of number portability, the B-number no longer refers to the subscription network/number range owner and the old rule “the number range of the called party reveals the tariff level” is no longer valid. One example of an IN service utilizing the subscription network/number range owner is a prepaid subscription. The prepaid subscription involves no billing in arrears but the account of the prepaid subscription is charged real-time during calls. In solutions based on an intelligent network, a prepaid subscription is charged in phases and usually at least two network nodes participate in the charging. The balance of the account is monitored in a network node controlling the service and called a service control point (SCP). SCP determines, for example, a reserved credit, which is the amount of money reserved from the balance for this call for a certain period of time. The amount of the reserved credit depends on a threshold, which is a network parameter usually expressed as time, and on a price of one time unit for this call. The price is usually determined on the basis of the B-number range owner/subscription network of the B-number. SCP sends the threshold to a service switching point (SSP). The service switching point is a call monitoring network node, for example a mobile switching centre. When for example the time indicated by the threshold has elapsed, SSP reports this to SCP and at the same time requests another threshold. However, the threshold determination described above can not be used with number portability, since the B-number does not necessarily reveal the subscription network/number range owner, and thus it can not be used for determining the time.
Another service similar to the prepaid service with a similar problem is a postpaid service with a limited bill, i.e. a postpaid subscription with a limit indicating the maximum allowed amount to be billed. It differs from the prepaid subscription in that the subscription allows access to chargeable connections or services until the bill reaches the limit.